The tectono-magmatic and subsidence evolution during lithospheric breakup in a salt-rich rifted margin: Insights from a 3D seismic survey from southern Gabon

Abstract

In this study, we present an interpretation of a 3D depth-migrated dataset imaging the Ocean Continent-Transition (OCT) of the southern Gabon margin. Located in an area where salt is not strongly overflowing the OCT, it allows the geometrical relationships of salt deposition and its relative timing to breakup to be determined. The OCT is formed by extensional propagators floored by exhumed mantle limited by either fault-bounded upper-crustal or pre-salt sediments and/or magmatic additions. Based on our interpretations, we conclude that the pre-salt sequence was deposited over a wide area spanning crustal thinning to the onset of hyperextension and exhumation. The relative timing of salt deposition is constrained by the observations that detachment faults break away in the pre-salt sequence and that some of these faults also strongly offset the base salt and are linked to the creation of accommodation space in the salt. This is direct proof that salt was deposited during mantle exhumation. Salt deposition is interpreted to have occurred at variable depths. Evaporites formed approximately 1 km below sea level in the north, above a 4–6 km thick pre-salt sequence, and as deep as 2 km in the south. In this area, salt is interpreted to directly overlie exhumed mantle, but whether it was deposited on mantle or juxtaposed during exhumation remains more debatable. Space created in the faulted outer trough was filled partly by autochthonous salt and possibly also by downslope flow. Rifting continued after the cessation of salt deposition, followed by magmatic additions in the outer high and ultimately the onset of seafloor spreading and the formation of Penrose oceanic crust.